The conventional method for delignifying and bleaching lignocellulosic pulp has been to employ a variety of multi-stage bleaching sequences, for example, 4, 5, or 6 stages, which traditionally have been based on the use of chlorine and/or chlorine dioxide in the bleaching or brightening stages.
In practice, intermediate alkaline extraction stages with caustic are used between the chlorination and bleaching stages. When applied immediately following the chlorination stage, denoted as CE, the first alkaline extraction stage is used to solubilize and remove a major portion of the chlorinated and oxidized residual lignin (chlorolignin) that is retained in the chlorination stage pulp, remove fatty acid-esters and resins present in the pulp, and also remove hemicellulose. It is an integral part of any multi-stage bleaching sequence. The resultant chlorinated-extracted pulp contains only 0.5% to 1.0% residual lignin which is amenable to further oxidation and brightening in later stages without excessive bleach chemical usage. However, while the alkaline extraction stage prepares pulp for subsequent bleaching, its immediate effect is to darken the remaining pulp impurities relative to chlorinated pulp.
With the advent of stricter environmental regulations designed to abate water and air pollution problems associated with chlorine--containing bleaching chemicals, coupled with their high cost and extensive recovery systems needed for their removal from the effluent streams, the paper industry has directed its attention to other bleaching chemicals which might avoid these problems.
One of the major areas of research and development in the last number of years has been directed to the use of oxygen as a delignifying agent and bleachant in various pulp bleaching sequences. One bleaching innovation that has been widely acclaimed is the use of oxygen in conjunction with a conventional alkaline extraction stage, denoted as Eo, immediately following a chlorination stage. It has achieved widespread commercial implementation in the last number of years. D. W. Reeve in TAPPI: 67(4), 143 (1984) provides a partial tabulation of worldwide Eo installations, and Enz and Hallenbeck presented a detailed description of the effects an Eo-stage has on bleach plant operations and pulp quality (1983 Pulping Conference, Houston, Tex., November 1983, pp. 309-313).
The principle action of oxygen in an alkaline extraction stage is to partially delignify and brighten the pulp compared to a conventional non-oxygen reinforced caustic extraction. Typically, in mill practice, the lignin content of the pulp is decreased by 18-25% of the lignin normally remaining in a conventional softwood kraft pulp subsequent to a CE treatment, as measured by Tappi standard method T214. In the case of hardwood kraft pulps, the delignification usually does not exceed a 15-20% decrease in lignin content when compared with a conventional CE bleached pulp. The brightness of both softwood and hardwood pulps is increased by about 4-6% GE points when an oxygenated alkaline extraction stage is employed compared to an alkaline extraction stage in which oxygen is not used.
While the use of oxygen in an alkaline extraction stage materially increases delignification and brightness, it adversely affects pulp viscosity which represents a drawback to its use. Typically, a softwood or hardwood kraft pulp extracted in the presence of oxygen has a viscosity which is 2-3 cps lower than a pulp which has been conventionally extracted.
Accordingly, it is an object of the present invention to provide a method for increasing the amount of delignification and brightening provided by an oxygen-alkaline extraction stage (Eo) without incurring additional viscosity losses.